(a) Field of the Invention
The present invention relates to copper fine powder which has a considerably low electrical resistance in its powdery state, is excellent in packing properties and has sharp particle size distribution as well as a method for preparing the copper fine powder. More particularly, the present invention pertains to copper fine powder used in making a copper paste which is suitably used for forming an electric conductor pattern on a resin substrate mainly used as an electronic circuit, in particular, a multilayer printed wiring board as well as a method for preparing such copper fine powder.
(b) Description of the Prior Art
Conventionally, copper fine powder has been used as a raw material for the so-called paste to be fired, i.e., a paste which is applied onto a substrate for electronic machinery and tools, such as a glass or ceramic substrate by the screen printing or the direct drawing and then fired to give a thick film.
Recently, there has been desired for the development of such electronic machinery and tools each having a higher packaging density along with the development of high speed, digital electronic machinery and tools for the purposes of miniaturization and weight-reduction thereof and for further functionalization thereof. Nevertheless, circuit boards have conventionally been prepared by the through hole method wherein through holes are formed in a substrate by drilling operations and the substrate is then plated. There are technical limits in these conventional methods and they have not been able to satisfy the foregoing requirements.
As a means for satisfying the foregoing requirements, the development of a multilayer printed wiring board having a via hole (VH) structure has become of major interest lately. In this respect, the via hole structure may cope with not only the reduction in the substrate size, but also automatic design for distributing wires and digital high speed processing of signals.
The multilayer printed wiring board having such a VH structure has been produced by filling the holes formed in the substrate with a solvent free type heat-curable conductive paste which comprises copper fine powder, a resin and a curing agent; sandwiching the substrate between two copper foils; and then subjecting the assembly to heat and pressure. The heat-curable conductive paste used for the production of the printed board must further satisfy such requirements that the copper fine powder as an electrical conductor should have further excellent characteristic properties such as conductivity and packing properties since any evaporation of organic binders and solvents does not cause, unlike the conventional paste to be fired.
With regard to the copper fine powder, various attempts have conventionally been done to improve the copper fine powder so that it has characteristic properties, which make the use thereof in the solvent-containing paste to be fired favorable, for instance, the shape, particle size, particle size distribution and tap density of the copper fine powder, but there have not yet sufficiently been investigated characteristic properties of the copper fine powder which make the use thereof in the solvent-free heat-curable conductive paste favorable. Accordingly, there has not yet been proposed any copper fine powder capable of satisfying the requirements for the characteristic properties of the copper fine powder which permit favorable use thereof in the solvent-free heat-curable conductive paste.
There have conventionally been proposed various kinds of methods for preparing copper fine powder such as mechanical pulverization methods, atomization methods, electrolysis methods, evaporation methods and wet-reduction methods. The wet-reduction methods have been recognized to be preferred methods for preparing copper fine powder for use in the paste to be fired and, in particular, several reduction methods using hydrazine have conventionally been proposed as appropriate means for preparing copper fine powder having a particle size on the order of 0.1 to 100 .mu.m.
Typical examples of such methods are production methods as disclosed in, for instance, Japanese Un-Examined Patent Publication (hereinafter referred to as "J. P. KOKAI") Nos. Hei 2-294414, Hei 4-116109 and Hei 4-235205.
J. P. KOKAI No. Hei 2-294414 discloses a method for preparing copper powder which comprises the steps of adding an alkali hydroxide and a reducing sugar to an aqueous copper salt solution in the presence of at least one compound selected from the group consisting of amino acids and salts thereof, ammonia and ammonium salts, organic amines and dimethylglyoxime to thus precipitate copper suboxide particles; and then reducing the copper suboxide particles with hydrazine.
In addition, J. P. KOKAI No. Hei 4-116109 discloses a method which comprises reducing an aqueous copper salt solution into metal copper particles through copper hydroxide and copper suboxide. In respect of the method, this patent also describes that a reducing sugar and then a hydrazine reducing agent are added to the aqueous copper salt solution after adjusting the pH value of the solution to a level of not less than 12, that the temperature of the reaction solution is controlled to not less than 60.degree. C. prior to the addition of the hydrazine reducing agent and that a chelating agent such as a Rochelle salt, an amino acid, ammonia or an ammonium compound is usable to maintain stable dispersion of copper (II) ions in the aqueous solution.
Moreover, J. P. KOKAI No. Hei 4-235205 discloses a reduction method identical to that disclosed in the foregoing J. P. KOKAI No. Hei 4-116109 except that the former further comprises the step of adding a protective colloid to the aqueous solution in portions.
The foregoing Japanese Un-Examined Patent Publications listed above disclose that the copper fine powder prepared by the method disclosed therein is, for instance, characterized in that it has a narrow particle size distribution and a small particle size, but the copper fine powder has still been insufficient for use as a raw material of heat-curable conductive pastes for filling up VH's since the particle size distribution thereof is still wide and there is observed a bias in the particle size towards the side of smaller size.